Shelf ord. — Physiological Characters and Depth 193 



of Sir Edward Forbes to divide the margins of the sea 

 bottom into several belts, the uppermost of these is com- 

 monly known as the shore belt and reaches from the 

 level of the usual high tide to the average of low tides 

 which is about three feet above the mean low tide of 

 the U. S. Navigation Charts. The reason for this lower 

 limit not agreeing with the mean low tide lies in the 

 fact that the usual low tide is considerably above mean 

 low tide level so that animals living within about three 

 feet from mean low tide are exposed out of water only 

 for a few hours during a brief period once a month. A 

 growth of Ulva which reaches up to this level covers the 

 stones quite completely at such times so that the animals 

 are not fully exposed. 



Immediately below this is the Laminarian Belt which 

 is characterized by broad leaved algae. The algae shelter 

 animals from light and enemies. The lower limit of this 

 belt is the lower limit of light for green algae. The 

 Laminarian Belt extends from three feet above mean 

 low tide to a depth of about sixty feet. 



The belt below this is commonly known as the Coral- 

 line Belt because of the presence of Coralline Algae. 

 For convenience it may be subdivided into the Coralline 

 which reaches to 300 feet and the Subcoralline from 300 

 feet to about 600 feet. It is characterized by a very 

 short daylight period and faint light at all times. 



The advantages of the Puget Sound Marine Station 

 locality for the study of physiological difference between 

 animals from different depths lies in the fact that the 

 abrupt shores make it possible to get animals from 

 several different depths ranging from feet to 540 feet 

 (165 meters) within a few moments and submit them 

 to experimental conditions within a short period. 



II. — RESISTANCE TO HIGH TEMPERATURE. 



The following selected results of comparison of the 

 resistance of animals of the same species from different 

 depths to fatal conditions will serve to illustrate the 

 whole series of experiments. 



